Existing rapid prototyping technologies for light curing resins mainly comprise SLA (stereolithography apparatus) and DLP (Digital Light Processing, digital light processing technology). The DLP rapid prototyping technology uses simple equipment and is much faster in prototyping than the SLA rapid prototyping technology; therefore, the DLP rapid prototyping technology becomes more popular in the market. In addition, the DLP-based mask projection stereolithography technology features high precision, especially applicable to such fields as lost-wax casting of metal jewelry. However, a projected image of the DLP-based mask projection stereolithography is a bitmap converted from a vector diagram. Referring to FIG. 1, a vector section of a three-dimensional model is a circle in this example and is converted into a bitmap 10 composed of a great quantity of pixels. In addition to a main body area 101, a boundary area 102 not belonging to the vector section is comprised in the bitmap during a conversion process while a boundary area 103 belonging to the vector section is excluded. In this case, a DLP mask projection area is different from a three-dimensional model vector section area, resulting in deformation of a prototyped object to some extent. For a small-size three-dimensional object, an edge distortion of a bitmap means a greater edge distortion of a vector section, thereby affecting precision of object prototyping.